This invention relates generally to protection zone selection systems for microprocessor-based bus protection relays, and more specifically concerns protection zone selection systems using graph/matrix theory.
In power system networks, a busline (also referred to as a busbar) provides a connection capability for various electrical circuits, including electrical generation, transmission and load circuits. If a fault occurs on a particular bus, the circuit or circuits which supply the fault current to the busline must trip their associated circuit breakers to isolate the fault from the other circuits connected to the busline. A bus fault may result in significant loss of service and, hence, busline arrangements are typically designed to minimize the number of circuits which must be opened in the event of a fault on the bus.
For various reasons, including improvements in energy availability and improved flexibility in busline system operations, busline arrangements have become more complex, with more circuits and individual busses involved in a given arrangement than previously. Such complexity requires in turn more sophisticated busline protection arrangements involving selection of protection zones for the bus in accordance with the particular configuration of the busline. It is important that the protection zone selection system operate such that the protection zones correlate appropriately with the busline arrangement so that the protective relay for the bus operates only for a protection zone fault.
In the past, both electromechanical and analog electronic relays have been used for bus protection in electric power systems. Most uses of such relays for bus protection were and still are for relatively simple bus arrangements. With the more sophisticated, multiple bus arrangements, electromechanical relay approaches are particularly complicated, unreliable and expensive.
Analog electronic bus relays, on the other hand have been more widely used than electromechanical relays to protect the more complex bus arrangements. Various approaches have been used involving analog electronic bus relays.
One approach uses directional comparison with a current differential scheme. In operation, the relay switches current direction signals for directional comparison and switches current transformer (CT) secondary current signals for the differential approach. However, the CT secondary current switching approach for protection zone selection may result in a hazard by virtue of an open CT secondary circuit.
In another analog approach, electronic relay approach a medium impedance differential bus relay has been developed which avoids the hazards of the first approach. Switching occurs on the secondary of ratio matching CTs, eliminating an open circuit in the main CT secondary. This arrangement still requires switching trip circuits, however, even though it does eliminate CT switching.
In still another approach, auxiliary relays are used to represent the various disconnect switches in the bus arrangement. Modular relays are designed to replicate the station bus components. The replica framework can then be arranged in different configurations to provide varied possible protection zone schemes for evaluation relative to different bus arrangements. However, the design and implementation of replica devices is considered to be a major drawback.
In the present application, a new approach for protection zone selection is disclosed using graph/matrix theory with microprocessor-based bus relays. This new approach eliminates the disadvantages associated with the various known approaches described above and is applicable to any bus arrangement in power system stations.
Accordingly, the present invention is a system for protection zone selection for a power system bus, comprising: means for receiving information concerning a bus arrangement in a power system, including the number of busses in the arrangement, and graph-based information concerning the arrangement, including the identification of vertex and edge portions of the bus arrangement, wherein said edges include disconnect circuit breakers and current transformer (CT) branches and said branches include the busses and convergence points between disconnect branches and circuit breaker-CT branches; means for receiving the operational status of the disconnect branches and the circuit breakers, said operational status defining a selected operational configuration of the bus arrangement; a processor for establishing an incidence graph arrangement indicative of said vertexes and edges of the various vertexes, and for altering the established graph matrix based on an actual operation status of the disconnect switches and the current breakers; and means for selecting a series of protection zones covering the busses in the bus arrangement from the graph arrangement.